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Has a commercial jet airliner ever stalled on takeoff / at low-altitude?
In general aviation, a common reason for fatalities is low altitude stalls on takeoff or practising go-arounds.
Has there ever been an incident where a commercial jet airliner stalled on takeoff / at low-altitude?
(Excluding the one where the tank wasn't secured, pretty sure that wasn't commercial anyway.
Also excluding the MAX 8 incidents).

Not an answer because I don't know if it was definitely a stall, but possibly Bek F100 at Almaty on Dec 27th 2019 http://www.avherald.com/h?article=4d127dc6&opt=0

@DaveGremlin, basically all icing accidents as well as the loss-of-control-in-engine-out-situation (mostly turboprops) end up as stalls. However in these cases stall is not the primary cause, but a result of other condition causing loss of control. For accidents where pilots not recognizing or handling stall the West Caribbean 708 comes to mind, but that was in cruise, so it does not satisfy the criteria of the question. I am sure there are others that do, but I can't remember any specific from top of my head.

No reason to explicitly exclude the MAX incidents. They never stalled.

"In general aviation, a common reason for fatalities is low altitude stalls on takeoff or practising go-arounds."  That is not correct.  Most stalls are during the landing phase, when the pilot turns too steeply trying to line up with the runway. During takeoff, one has full power applied and is merely climbing straight out - and you have to pull the stick/yoke reeeeally far back to stall with full power - releasing the back pressure just a little bit will stop the stall from developing.  There is the occasional crash during takeoff, typically associated with an engine problem.

@CrossRoads Thanks for the correction, appreciate it. Please feel free to edit the question if you like.

There are two fairly recent airliner crashes I can think of that were the direct result of an aerodynamic stall, one is Air France 447 and the other is Colgan Air 3407.
AF447 was at cruise height when the incident began, so that one does not fit with your question.
Colgan3407 was landing, so was pretty much "low level" (Does "jet" permit a turboprop?).

Following the clearance for final approach, landing gear and flaps (5 degrees) were extended. The flight data recorder (FDR) indicated the airspeed had slowed to 145 knots (269 km/h).[3] The captain then called for the flaps to be increased to 15 degrees. The airspeed continued to slow to 135 knots (250 km/h). Six seconds later, the aircraft's stick shaker activated, warning of an impending stall as the speed continued to slow to 131 knots (243 km/h) [...]
In its final moments, the aircraft pitched up 31 degrees, then pitched down 25 degrees, then rolled left 46 degrees and snapped back to the right at 105 degrees. Occupants aboard experienced g-forces estimated at nearly 2 G's. The crew made no emergency declaration as they rapidly lost altitude and crashed into a private home at 6038 Long Street,[24] about 5 miles (8.0 km) from the end of the runway

Going back in time, we can probably find a few more - some of the older examples may stretch what you consider "commercial aviation" by todays standards, and some most certainly were not "Jet" engined, but for the sake of completeness here goes:

1923 Daimler airways DH34 crash

The flight was a scheduled domestic passenger flight from Croydon to Manchester. It was also reported to be carrying mail,[2] although this was later denied by the General Post Office.[3] While flying over Buckinghamshire, a storm was encountered. Witnesses stated that an engine stopped, but was then restarted. It appeared to them that an emergency landing was going to be made at Ford End, Ivinghoe when the aircraft dived to the ground and crashed at Ivinghoe

Another DH34 a year later

The aircraft took off from Croydon Airport on a scheduled international passenger flight to Le Bourget Airport, Paris. Witnesses described the aircraft as flying low over Purley before nosediving to the ground

American airlines flight 1

American Airlines Flight 1, dubbed "the New Yorker",[1] was a regularly scheduled, multiple stop flight from La Guardia Airport to Chicago Municipal Airport. It had intermediate stops at Newark, New Jersey; Buffalo, New York; Detroit, Michigan; and South Bend, Indiana. On October 30, 1941, on the flight's leg between Buffalo and Detroit, the American Airlines Douglas DC-3-277B operating the route crashed into a wheat field approximately one half mile east of the town of Lawrence Station, Ontario

Llandow air disaster

the Avro Tudor was approaching runway 28 of Llandow aerodrome at an abnormally low altitude with the undercarriage down. The pilot attempted to correct the descent by increasing the power of the engines and brought the plane up. The aircraft rose steeply to 100 m (300 ft) attaining a nose-up attitude of 35 degrees to the vertical, and then the aircraft stalled

Aeroflot Flight 513

The aircraft was de-iced before takeoff. In the cockpit, the check captain observing the trainee's performance sat on the right; the trainee sat on the left. The captain and first officer remained in the cabin and did not assist the trainee and check captain during takeoff. Flight 513 took off from the runway at a bearing of 100°. At an altitude of 40–50 meters the angle of attack increased to the point of causing a stall. The Tu-124 never recovered from the stall and crashed into a field of snow

Delta flight 1080

During the takeoff, the Lockheed L-1011's left elevator became stuck in a fully upwards position, leading to the aircraft pitching up aggressively and causing the aircraft to lose speed and nearly stall. The pitching force, unable to be overcome by fully pushing the control column down, was counteracted by reducing the thrust on the L-1011's wing engines but not the tail engine. The differential thrust pitched down the nose of the airliner and allowed the pilots to land the aircraft.

China Eastern Airlines Flight 5210

On 21 November 2004, just two minutes after take off, the Bombardier CRJ-200ER fell from the sky and crashed into a lake in Nanhai Park, next to the airport, killing all 53 people on board and 2 more on the ground. [...] An investigation by the Civil Aviation Administration of China (CAAC) revealed that the plane had not been de-iced by the ground crew while it was parked on the tarmac.

Regional Airlines Flight 9288

On approach to Varandey Airport, the crew allowed the An-24RV's speed to drop and its nose to rise until in stalled. At 13:53, the aircraft struck a hill, crashed about 5 kilometers (3.1 miles) from the airport, and burned, killing 28 people (two crew members and 26 passengers).

West Caribbean Airways Flight 9955

The aircraft, a Let L-410 Turbolet, had just taken off from El Embrujo Airport at 9:50, when the left engine flamed out. The crew continued with the takeoff, but the speed of the aircraft decreased rapidly. The aircraft then banked dangerously too far to the right and stalled. The aircraft crashed into a mangrove forest, located just 113 metres (371 ft) from the airport runway.

Turkish Airlines Flight 1951

The crash was caused primarily by the aircraft's automated reaction, which was triggered by a faulty radio altimeter. This caused the autothrottle to decrease the engine power to idle during approach. The crew noticed this too late to take appropriate action to increase the thrust and recover the aircraft before it stalled and crashed.

TransAsia Airways Flight 235

Two minutes after takeoff, the pilots reported an engine flameout. Flight 235 climbed to a maximum height of 1,510 feet (460 m), then descended. The other engine, still working, was shut down mistakenly.[1][2] Immediately before crashing into the river, it banked sharply left and clipped a taxi travelling west on the Huandong Viaduct

So, yeah, quite a few actually!

The Regional Airlines 9288 stands out as one where no other cause beyond pilots mishandling the plane was found. Another such might be Ethiopian 409 (though it took some time after take-off to the fatal mishandling). Yemenia 626 also comes close, though that had a bit of contribution from the system.

The Delta 1080 crew "did well".

@RussellMcMahon The TransAsia crew, not so much.

Does BA flight 38 count? The pilots reported the stall alarm sounded right before the crash.

Two other catastrophic accidents during takeoff phase occurred on flight testing for transport category aircraft, though both of them are business jets:
1. Bombardier Challenger 604, 2000, Wichita
Aggressive takeoff rotation led to fuel migration and shifted the aircraft CG aft of the allowed limits. The combined effect of the large initial rate of rotation and the pitching moment from the rearward CG movement caused the aircraft to stall at low altitude, with insufficient altitude to recover. The right wing rolled and impacted the ground first and the aircraft exploded on impact.
2. Gulfstream G650, 2010, Roswell
The design target V2 speed for the G650 was erroneously calculated, which was lower than physically achievable. Furthermore, the engineering team overestimated the stall AOA of the aircraft in ground effect, resulting in lack of stall warning/crew awareness as the aircraft approaches stall in ground effect. In an effort to achieve the target V2 in one-engine-inoperative flight tests, increasingly aggressive rotation technique was used, the last of which resulted in a stall at low altitude, with insufficient altitude to recover. The right wingtip contacted the runway; the aircraft departed the runway, impacted a structure and burst into flame.

There are a couple I can think of off-hand from the last couple of decades (in addition to the ones already listed in other answers):
1. National Airlines Flight 102
I suppose whether this one counts depends on exactly what definition of "airliner" you're using. Certainly, the 747-400 is normally considered an airliner, though this was its cargo variant. I would consider it an airliner, but not a passenger airliner.
At any rate, this flight in April 2013 crashed shortly after departure from Bagram Air Force Base in Afghanistan. At least one armored vehicle in the cargo deck was improperly restrained, broke free, and fell against the aft bulkhead very shortly after takeoff. This moved the CoG aft and also damaged flight controls. As a result, the aircraft pitched up into an unrecoverable stall at low altitude and crashed shortly after takeoff.
2. Air Midwest Flight 5481 / U.S. Airways Express 5481
This was was a 19-seater turboprop operated by Air Midwest for U.S. Airways Express. In this incident in January 2003, the NTSB found that the aircraft had been loaded 580 pounds above its maximum take-off weight and that its center of gravity was 5% aft of its allowable limit. The NTSB determined that this was due to both the passengers and their luggage being heavier than the average values that the FAA allowed the airline to use for computing weight and balance.
Additionally, improper maintenance had been performed on the aircraft that resulted in the elevator authority being significantly reduced. The combination of these two issues led to the aircraft becoming uncontrollable immediately upon takeoff. It rapidly pitched up, eventually reaching 54 degrees nose-up attitude. After a rapid climb, the aircraft entered an unrecoverable stall and crashed into a maintenance hangar approximately 35 seconds after lifting off of the runway.

Sounds similar to the De Havilland Comet 1's other problem.

On 26 October 1952, the Comet suffered its first hull loss when a BOAC flight departing Rome's Ciampino airport failed to become airborne and ran into rough ground at the end of the runway.
...
Both early accidents were originally attributed to pilot error, as over-rotation had led to a loss of lift from the leading edge of the aircraft's wings. It was later determined that the Comet's wing profile experienced a loss of lift at a high angle of attack, and its engine inlets also suffered a lack of pressure recovery in the same conditions.
[quote from Wiki link above]

If the wheels never actually left the ground, this wouldn't count as "low altitude", however it looks like the correct conditions for a stall even if no loss of altitude was physically possible.

One of the lucky ones :-(

@RussellMcMahon: Only the first one; the second failure-to-take-off accident (in Karachi) killed everyone on board.

The 1972 crash of BEA flight 548 in Staines, near Heathrow Airport, London, was, and remains, the most lethal flying accident (excluding terrorism) in the UK. It was a significant incident with respect to raising issues of crisis management in the cockpit, and in bringing about the use of cockpit voice recorders. The proximate cause was a decrease in the airspeed after the premature retraction of the leading-edge droops, but in addition, during the enquiry, there was considerable dispute over the proposition that the Captain could have been undergoing an acute coronary or arterial condition (frequently, but somewhat inaccurately, described as a 'heart attack') at the time of the crash.
https://en.wikipedia.org/wiki/British_European_Airways_Flight_548
Another significant British case, though not initiated at low altitude, was that of the first prototype BAC One-Eleven, which entered an irrecoverable deep stall while undergoing stall testing. This brought to the forefront concerns about the propensity for this scenario in rear-engined, T-tailed jets, and led to the installation of stick-pushers on this and similar aircraft.
https://en.wikipedia.org/wiki/1963_BAC_One-Eleven_test_crash
A similar crash, involving a Hawker-Siddeley Trident (the same type as in the Staines crash), occured during a post-manufacture certification test flight, while the stick shaker and pusher were disabled.
https://en.wikipedia.org/wiki/1966_Felthorpe_Trident_crash


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Can I add ratings to a NNPL?
I have been speaking to a couple of local flying schools. Originally, I was going to train on a Piper Tomahawk to obtain an LAPL. But after explaining to one of them my goals, (just flying for fun, no career aspirations) they suggested an NPPL would be more appropriate.
They explained that it would cost about half the amount (in money and time) to obtain and that their Ikarus C72s are safer, easier and more fun to fly anyway.
I am on board with all of this but I think I would like the freedom to fly in IMC in the future, they said I can add an instrument rating to an NPPL and that I can convert to an LAPL in the future if I want to anyway, is this the case? I have looked on the CAA website, but it's all a bit confusing right now.

From what I've read, you cannot have an IR(R) (instrument rating) on an NPPL or LAPL. Both the NPPL and LAPL are VFR only licenses. NPPL also cannot have night-flying privileges.

That's what I said when you posted on this the other week; just do an NNPL.  99% of private pilots fly alone or with 1 passenger, day VFR.  An instrument rating is a waste of money if you aren't going professional or are planning to get a sophisticated ac.  You aren't going to want to go IFR anywhere in a basic single engine airplane.  To have any real utility, an IFR equipped airplane needs to have deicing or it is stuck below the freezing level, which in the UK and northern Europe keeps you below 5000 ft much of the year and pretty much grounds you in winter... unless it's VFR.

Also, if you really want to become a superior pilot from a skills perspective, do a glider license first.

@JohnK Really feel you're overstating some points there. While much doesn't matter, Private flying is hardly constrained to one passenger and a night rating is super useful in Winter due to early sunset. However, I feel your biggest oversight is the IR(R) which most definitely doesn't need a FIKI aircraft to take advantage of. I bet most UK GA pilots rarely venture 5k+ anyway.

Perhaps, but the day VFR rec/sport/NNPL license statistically covers the vast majority of pilots out there.  I agree a night rating is good to have but if the idea is to tour Europe confident you can get there 99% of the time because of an instrument rating, it's going to be disappointing.  In any case, first things first and the NNPL gets him 90% to where he wants to go, and from there, play it by ear.

@JohnK I broadly agree, but the NPPL is quite restricted in the UK. For example, next year, you won't be able to fly EASA aircraft - for the cost, I think the PPL or at least LAPL is worth it. Bearing in mind we're a short hop from France and Ireland.

@JohnK Thanks for the feedback but I have a couple of questions. An instructor told me on the weekend that he believes the NPPL will be retired entirely soon, making it invalid. Also, you say instrument rating is a waste of time and money but... it can save your life! I don't want to be in a '178 seconds to live' scenario, wishing I'd done my instrument rating :)

@Dan They have been saying that for about 5 years I hear, and keep pushing it back.

@Cloud Saying what, sorry?

@Dan That the NPPL will not allow EASA aircraft to be flown

I'm saying an instrument rating is a waste of money if you don't plan to actually use it, unless you enjoy dropping 10 or 15 grand just to know stuff.  When you take an IFR course, by which time you already have a PPL and night rating, you already know how to fly on instruments with the basic skills necessary to get yourself out of trouble.  The IFR course is on how to operate in the ATC system, navigate, plan, do approaches etc.

Anyway, if the NNPL is revoked and not replaced with something else, you would then just carry on with training as if it never existed in the first place.  You don't have to start over.  Stop over thinking stuff to death and just go have fun.

Available Ratings:

You can add class ratings to the NPPL to allow you to fly microlights, self-launching motorgliders (SLMGs) and simple single-engine aeroplanes (SSEAs).

The above are the only available ratings for an an NPPL. An NPPL (and LAPL) is strictly a VFR license, therefore you could not get rated for IMC. However, you can continue your training to receive your PPL, at which point you could recieve a rating for IMC, which would be your IR(R), or Instrument Rating Restricted.
Most flight instructors will provide basic IMC training just in case you find yourself in a bad situation, which can happen to even the most cautious pilots.

this is where the confusion comes in. I don't think any hours from microlights count towards a LAPL. So a 1,000 hour NPPL pilot would still need to do minimum hours and training in a Class A to get an LAPL or PPL.... I think.

Are you sure that an LAPL can get an IR(R) or IR rating? The things I've read indicate otherwise, although I haven't gone through the EASA regulations to verify.

@RonBeyer No, my bad - edited to include LAPL as VFR license. You can recieve basic IMC training with your LAPL though.

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Using pure O2 for jet engine combustion
In addition to my previous question, I am mainly concerned with burning all the fuel.

Why isn't the combustion process using all of the Oxygen in the air?
Is it because other gases are in the way and it isn't in the right place to help the reaction?
Are engineers limiting the amount of fuel into the process because they know some of the Oxygen won't be available for combustion due to its proximity(lack thereof) to the fuel?
How much fuel remains un-burnt in a modern jet engine?
I thought that an after-burner was meant to use the fuel that somehow got through the whole process without combusting?
Wouldn't using pure O2 allow for a complete fuel burn?
I do get that the added weight may not make it worthwhile since we need to weigh the value of complete burn against other non-monetary concerns(like pollution).

Finally, I assume the 2000-2500 degree C temp mentioned (in this answer) in the last post is to actually burn Oxygen. Although I know Oxygen is not flammable, I am sure everything does burn at some point. I do not want to actually burn the Oxygen.

So far as I can tell this is covered by this answer to your previous question. If you want to better understand how oxygen and flammability relate, it might be better to ask that specific question in Physics.se or Chemistry.se

@RedGrittyBrick: It seems Jon was not completely happy with the answer, and he should ask until we can provide a satisfying explanation. Besides, the answer you cite did not get all the facts right.

Ugh, voting to re-open against my better judgement - There are too many questions here, perhaps making this too broad.

@CGCampbell If you believe that the question should be closed, don't vote to re-open it! Reopening a question that was closed for the wrong reason, just so it can be closed for the right reason isn't a productive use of anyone's time.

If you're going to carry all your propellant and oxidizer, well, you now have a rocket engine...

Oxygen cannot burn as burning (combustion) is the chemical reaction of a material with Oxygen. Basically, all the fuel that goes into the combustion chamber is burnt. If all the Oxygen was used for combustion there could be no afterburners without adding fresh air into the exhaust. Most of the air that goes into a straight jet is actually used to cool the combustion chamber, to protect it from the flame which burns at about 2100°C (https://en.wikipedia.org/wiki/Adiabatic_flame_temperature) (fuel oil flame temperature)

The compression in the forward part of a jet engine heats the air, and combustion raises its temperature again. In order to achieve efficient operation, this heating must be restricted. If air is heated above approx. 2,000 K, adding more energy will result in dissociation of the gas with less further heat increase. Since thrust is produced by expanding air through heating, raising combustion temperatures above 2,000 K will result in less thrust increase for the amount of fuel consumed. The most advanced engines (F-119 and EJ-200) have turbine entry temperatures of 1800 K, and civilian engines operate a few 100 K below that. The number of 2000 - 2500°C (2300 - 2800 K) you got from a previous answer is way too high.
Oxygen starts to dissociate already between 2,000  and 4,000 K, depending on pressure, while Nitrogen will dissociate mainly above 8,000 K. The 2,000 K figure above is a soft limit, but this temperature is also a challenge for the materials of the combustion chamber and the turbine, and techniques like film cooling are mandatory. As you can see, using pure oxygen would greatly increase the efficiency loss due to dissociation once technology allows to run the turbine at higher temperatures than 1800 K. It would be better for jet engines if air had a lower oxygen content.
To make the best use of the fuel, much more air than would be needed for stochiometric combustion is used in a modern turbojet or turbofan core. The idea is to heat the gas just as much as technically and economically possible.
Too much oxygen does create problems, mainly by raising the amount of nitrous oxides a jet engine produces. However, it helps to achieve largely complete combustion, even if some of the reaction happens not in the combustor, but at later stages. Combustion is never fully complete, and the remaining unburnt constituents are proportional to an inverse exponential function over time.

" It would be better for jet engines if air had a lower oxygen content." - Does that mean it would help to feed back part of the exhaust gas to the intake air?

"Too much oxygen does create problems" Not least of which is that it converts your "jet engine" into a "rocket engine".

@VolkerSiegel, the exhaust gas is already hot and therefore useless. More cold gas would help, but the only cold gas practically available is ambient air.

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Would converting a Lazair ultralight to 4 x 3 Hp engines and using the props as distributed propulsion produce a Clmax of 4.5?
Would converting a Lazair ultralight to 4 x 3 Hp engines ( 12 Hp total ) and using the props along the entire wingspan as distributed propulsion produce a Clmax of 4.5?
I understand the original Lazair had 2 x 5.5 hp engines, for a total hp of 11hp.
I understand the Lazair has no slats or flaps, so the clmax of it's wing would be about 1.5.
I also understand that distributed propulsion adds about a cLmax of 3 to a wing.
So, what would happen if you converted a Lazair to a 4 engine plane with 6' propellers across 100% of it's entire wingspan to generate distributed propulsion?
Would the clmax jump from 1.5 to about 4.5 or is 12 Hp way too low to produce an increase in lift of 3? If 12Hp is too low, how much Hp is required?

Even the Lazair 2 with 2 9.5 HP Rotax 185  only manages a modest 200 fpm climb with a 240 lb useful load.

It's been done.  Thing is, while you may get the theoretical jump to 4.5 Clmax, it's only for the 10 or 15% of the wing that is in the slipstream in the case of the Lazair and it's itty bitty props.
So overall, the effect in total CLmax of two extra engines blowing on the wings is negligible.  The 4 engine Lazair's performance was almost all from the doubling of thrust, with maybe a modest drop in minimum flying speed with power on with 4 engines vs 2, perhaps a knot or two.
On airplanes like large commuter turboprops with enormous 14 ft propellers that occupy a significant part of total span, the effect is much stronger.  Power settings have a near instantaneous and major effect on sink rate and you are keenly aware of the effect of the slip stream on total Cl.

So.. if you spaced out the 4 engines and had 6' props, covering 90% of the wing? Seems like it would jump the clmax from 1.5 to about 4, even with only 12 hp, no?

Well the velocity of the prop wash from 6 ft blades being driven by 12 hp would be pretty gentle so I think you'd lose a lot of the effect.

@Fred No, not if you leave the wing as it is. You need lots of camber, like slats plus slotted flaps with 40° deflection, plus slipstream. With the slipstream only, not much more than a lift coefficient of 1.5 will be possible. Maybe 1.8 for the full wing. But never 4.5.

Please don't run such experiments by yourself!
To change the thrust arrangement of an aircraft is to create a partially new aircraft. To do that, you have to know what you're doing and what, exactly, down to numbers, will happen.
Adding or spreading engines is not an automatic increase of lift coefficient, so the short answer is, no.

Here is some data from RC Groups about a person who built a 100% "scale" electric Lazair about 10 years ago, swinging two 32 inch x 12 props.
Thrust requirement was around 50 lbs.  Can't get around that, and loading up with high lift devices will only require much more.
Going back to the lessons we learned about Vbg and lowest drag configuration, particularly with an ultralight, high lift devices simply are not needed for an aircraft that (is very light) and will stop in a few feet anyways.  In fact, spoilers might be a better choice for this type (of powered glider).

Oh, if the Fiesler Storch only had retractable slats and flaps!

Miraculous increases in Coefficient of Lift do not come without increases in thrust, which invariably add weight in both engine and fuel.  Better to keep an ultralight aerodynamicly clean and focus on engine/prop thrust efficiency and weight savings.
All that being said, mounting the engines on the leading edges of the wings did produce an increase in performance$^1$, so, if you want to go with the original 5.5 hp chainsaw engines, 4 or even 6 of them may produce a fine multiengine ultralight.
It would do Convair proud to have 6 in a pusher configuration $^2$,$^3$,  which would help reduce drag as well as improve lift.  At least 20 horsepower (15 kW) seems to be optimal for reasonably good performance, especially with floats.
$^1$  Ultraflight Lazair, Development
$^2$ similar to this
$^3$ or perhaps more like this

Forget the lift coefficient for a moment. That jump by 3 was for one particular design - this is no magic number that applies everywhere.
Better think that you want to increase lift by a factor of three at a slower speed. Lift is the reaction force from deflecting a stream of air downwards. That stream can be approximated as the flow through a disk with a diameter equal to the wingspan which is 11 m in case of a Lazair. With a mass of 200 kg, standard atmospheric conditions in 500 m elevation (air density = 1.2 kg/m³) and your assumed maximum lift coefficient of 1.5, the flight speed at that lift coefficient is 12.8 m/s. For three times the lift coefficient speed could drop to 7.4 m/s before you stall.
Without flaps all that extra lift has to come from accelerating air downwards over the wing. This would require thrust far in excess of both induced drag and weight of the Lazair. In other words, if you only add propellers, without changing the wing's airfoil such that it would produce far larger deflection angles, the propellers and the wing have to be oriented up in order to make that slow flight speed possible. Thrust has only enough forward component to balance the aerodynamic force on the wing which is now mostly back due to the high angle of attack.
Using four propellers of 2 m diameter each would need 50 HP (= 12.5 HP for each motor) at least, assuming a generous propulsion efficiency of 0.75. If you quadruple engine power and have sufficient control authority for flying an angle of attack between 60° and 70°, go for it! Maybe you should check first, however, whether the beefed-up structure (so the props stay with the motors, and those stay with the wing) and the heavier engines still let you keep total mass at 200 kg.

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How do Alaskan bush pilots land and take off in 10'?
How do Alaskan bush pilots land and take off in 10'?
Just looked at a video of the Valdeez STOL competition with pilot's taking off and landing in 10'
How do they do it? And no, they don't have a headwind of 40mph. Headwind is usually 10-15mph which is a pretty common scenario for all of us.

As far as I know, these bush plane videos involve considerable headwinds. With enough headwind, you can go all VTOL.

Fred, there is a problem in your last paragraph. If he is already stalled, or even partially stalled, pulling up won’t arrest the descent.

The scores for 2019 (https://docs.wixstatic.com/ugd/c3e139_f7c42bbf4cff4a96bd816ca18c5a4464.pdf) suggest that in competition it's closer to 100' not 10' (assuming the numbers are in feet and not something else).

There are different classes. 100' sounds it's like for one of the commercial aircraft classes. The world record is about 9.5'.... ( yes, that is in feet)  A normal Zenith701 can take off in 50', 25' in a good headwind....

This question should be reopened .....

References to specific planes and specific distances (and specific headwinds) would be nice.  A stock 172 doing a 10' takeoff in the calm would require a magical explanation.  A heavily modified (extra power, lower weight) airframe with a 20kt headwind doing the same seems tricky, but less remarkable.

A STOL competition is not "bush pilots" to me.

I'm told most of the participants are Alaskan bush plane pilots They just get together once a year to have a little fun, no different than a car or motorcycle meet.


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If airfoils at low speed generate higher coefficient of lift, why is it not reflected in airfoil polars?
If airfoils at low speed are able to generate a higher coefficient of lift, why is this not reflected in airfoil polars?
I understand airfoils generate a higher coefficient of lift at low Re numbers.
So I entered the NACA2312 airfoil in javafoil and generated a lift polar for the airfoil from Re 5,000 to 10 million. There was no increase in lift at lower Re numbers...
Why not?

Where do you see that airfoil generates higher lift at lower Re?

No, they don't.
The Reynolds number is the ratio of inertial to viscous forces in a fluid. That means viscosity has proportionally more effect at lower Reynolds numbers. The boundary layer is proportionally thicker, friction steals more energy from the flow and lift is lower.
This answer contains a graph with a collection of empirical data, as does this answer. Here is another answer which covers both attached and separated flow. As you can see, Reynolds number effects mainly affect attached flow, and in a way that reduces lift for the same angle of attack; the more so, the closer you come to the stall angle of attack.
Just to save you from following all the links: See below for a plot of the venerable NACA 4412 from Abbott and Doenhoffs collection of airfoil data (picture source):

Note that the lift coefficient is plotted for Reynolds numbers R of 3, 6 and 9 million.

Above you state: The Reynolds number is the ratio of inertial to viscous forces in a fluid. That means viscosity has proportionally more effect at lower Reynolds numbers. The boundary layer is proportionally thicker, friction steals more energy from the flow and lift is lower. i.e. which states at lower speeds, lift is lower.

in the question of wether a wing can achieve a cLma of 6.8, you state: "Another practical limit is the maximum Mach number in the suction peak of all the airfoil elements. Once this reaches 1.58, no lift growth could be observed in experiments. This translates to a maximum for the product of Mach squared and pressure coefficient of -1.0. In other words: You need to fly very slowly in order to achieve high values of c".

That's where this question originates: You need to fly very slowly ( i.e. low Re number) to achieve high values of climax.  At what Re are we talking about?  i.e. Will a wing at Re 200k produce more lift than at Re 600k or Re 1 million?
.

@Fred: I understand – you expanded the validity of that statement too much. Slowly means first low Mach. At the same Mach you should increase Re for more lift. Going from 200 K (model aircraft) to 600 K (glider wingtip, slow speed) to 1 million (glider root, slow speed) will increase lift each time. And if you progress further, lift will increase even more (see diagram).


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How can a NACA2312 airfoil or similar have a camber location from 30 to 39% without a change in airfoil name?
How can a NACA2312 airfoil or similar have a camber location from 30 to 39% without a change in airfoil name?
Was fooling around in javafoil, and noticed I could go from 30% camber to 39% camber, without the NACA number of the airfoil changing!! What the heck?
What name is used to differentiate NACA2312 with 30% camber location and NACA2312 with 39% camber location?

The NACA airfoil numbers are coded as follows:
$$ \mathrm{NACA} \; M P XX $$

$M$ is the maximum camber divided by 100. In the example M=2 so the camber is 0.02 or 2% of the chord
$P$ is the position of the maximum camber divided by 10. In the example P=4 so the maximum camber is at 0.4 or 40% of the chord.
$XX$ is the thickness divided by 100. In the example XX=12 so the thiickness is 0.12 or 12% of the chord.

source: airfoiltools.com

In your case you changed the position of the maximum camber ($P$) from 30% to 39%. Both result in $ P = 30 / 10 = 3 $ and $ P = 39 / 10 = 3 $ (when using integers, no rounding is applied). The airfoil name therefore only tells you the position of maximum camber with a 10% accuracy.


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Is an 400+% increase in lift with distributed propulsion realistic?
Is an 440% increase in lift with distributed propulsion realistic?
I understand NASA swapped out the wings on their test plane with distributed propulsion from 17psf to 45 psf.  That's a factor of 2.6, which is an increase in efficiency of 260%!!
If the original wings have a cL of: 50% classic airfoil cL max of 1.5 + 50% slotted flap cl max of 2.7. that works out a wing cL max of about 2.1
So... cL of new wings must be a cL of 2.1 x 2.6 = 5.5, or an increase in cL of about 3.4.
S0... if I start with a foil like the Selig1233, with a max cL of 2.5, a slat adds .7 plus 3.4 for distributed propulsion, I'm looking at a total cL of 6.6.
A cL of 6.6/1.5 is a 440% increase in lift.
Is this realistic?
If not, what is?
Use would be for a small ultralight, max speed about 60mph.

Efficiency in what sense? For STOL? Sure. For cruise? Probably not unless cruising slowly is part of your mission.

Also, I see that you've asked 32 questions with zero accepted answer. We would appreciate if you can accept some of these answers, many of which took a while to do.

how do you accept answers?

There is a green check mark below the votes for accepting the answer. One question can have at most one accepted answer.


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Would a triplane be way more efficient if the wings intersected at the tips to create one wing tip vortex instead of 3?
Would a triplane be way more efficient if the wing tips were joined at the tips to create one wing tip vortex instead of 3, at RE 200-600k? At that low of a speed, I assume interference and form drag are minimal.
I understand biplanes and triplanes are less efficient than a monoplane due to wing root interference drag and wing tip vortices.
Yet..... I understand boeing/NASA are experimenting with a biplane/braced wing airliner configuration, which would have extra induced/inference drag.
So, to reduce those nasty wingtip vortices, couldn't one just intersect the tips?
I'm thinking, the top wing would have anhedral, a straight middle wing, and bottom wing with dihedral, meeting at the tips.

Are you thinking something like this? How does the wraparound wing design reduce drag?

It's not a matter of just blocking the flow of the vortice right at the wing tip by having another wing intersect it. There's actually a large flow field extending some ways out, only minimally influenced by wing tip gadgets.   It's like you blocked the fastest moving part of a river with a board stuck in the water, but all that slower moving water beyond the board still flows by.

Why do you expect the vortexes to do anything other than reinforce each other, leaving you with the same or larger vortex intensity of a monoplane and more structural complexity and weight?

Biplanes and triplanes are less efficient than monoplanes because for a given wing area, the monoplane will usually have a wider span. Span drives induced drag, of which the tip vortex is an indicator.

Well, those nasty wing tip vortices really can't be helped if you are creating lift.  Amazing that airliner designers in this century are still considering biplane vs strutted mono plane vs cantilever mono plane concepts in their proposals.
Bringing 3 wing tips together will create all kinds of air flow issues (remember to model at all possible AOAs), reducing efficiency.
The most efficient way to create lift remains a high aspect mono wing.  I would look to designers actually reducing sweep a little (around 15 degrees for subsonic) and try for as high an aspect wing as possible, just like sailplanes.
However, wing span constraints and support benefits for very large aircraft may yet see the return of the biplane, with very thin, high AR wings.  These could be swept a bit more at the tips, and/or endcapped or wrapped.  Fuel and landing gear could go in a more drag efficient longer fuselage.  Note mounting them in "classic" Tiger Moth style retains the stability benefits of wing sweep, which may be lost in the newer "Prandtl wraparound" proposal.
It seems some designers just can't bring themselves to make a 21st century high AR Jenny.  It might be worth a try.

sweeping high aspect ratio wings at the tip will create a torsion moment which those wings are poorly equipped to withstand. Better to avoid sweep.


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Does using wings with different wing loadings work the same as a swing wing, without swinging the wing?
Does using wings with different wing loadings work the same as a swing wing, without swinging the wing?
Was just wondering: we need the maximum co-efficient of lift at slower speeds at takeoff, but a much smaller co-efficient of lift at cruising speed.
For a triplane, wouldn't you want the top wing for flying and the other wings just for takeoff?
In other words:
you could make the top wing, a classic unsymmetrical wing like a NACA2415, set at around 5 deg angle of incidence, where I believe these airfoils are most efficient.
The other wings could be a wing with a high wing loading, like a high speed laminar foil, like the canard of a velocity, set at a incidence of 0 deg during straight and level flight.
In that way, the lower wings provide no lift, but also no induced drag ( drag due to lift) during straight and level flight, just like a classic tail airfoil.
During takeoff, you would have to rotate early prior to takeoff, ( like the Concorde) to get an angle of attack for all wings.
Say all 3 wings are the same size and you size your wings to get an overall wing loading of 7psf, which means you can land slow and short, but during cruise you would only be using 1 wing, so your effective wing loading is a very efficient 21psf. If it's gusty out, you could always land long and fast , using all 3 wings.  So this would be like a swing wing, without the complexity of actually swinging the wing.
Is this correct, at least conceptually?

Efficiency does not depend on wing loading but rather on wing span. At slow speed due to interference the three wings have induced drag like an only slightly wider single wing, and at high speed you are still getting the form drag of the two wings not taking part of lift generation. So I'd say no.

Actually I'm comparing a triplane with 3 similar wings to a triplane with one high lift wing and 2 low lift wings.  Not comparing a triplane to a monoplane.

Are you? I thought you were comparing it to swing-wing, which has only ever been realized as monoplane.

As for triplane with different lift coefficients on each plane compared to triplane with the same lift coefficient on each plane, the later is always more efficient, because the vertical spread does give you some increase in the affected air mass.

@ Jan:  I guess I wasn't clear.  What I'm asking is if the making the 2 lower wings high wing loading would improve the efficiency of a triplane, versus a triplane with all 3 wings the same.  I think the effect would be the same as a swing wing monoplane in wrt it would have about 66% less induced drag.

I've written the answer. No, it would be worse. Plus important part of what swing-wing does is reducing the form drag at high speed, which you are not doing at all.

For a triplane, wouldn't you want the top wing for flying and the other wings just for takeoff?

No.

Form drag is proportional to dynamic pressure and frontal area. By varying the lift coefficient between the planes you are not doing much to the frontal area, so the form drag is roughly the same.
Induced drag is proportional to square of lift and inversely proportional to square of span and dynamic pressure. By generating three times more lift with one of the wings, you are making it produce more than tree times the induced drag.
It is not all the nine times the basic equation suggests, because the efficiency is still significantly reduced by interference when all the wings are producing lift, but it is still worse, and it is worse whenever the lift is not distributed uniformly (maybe the interference makes a slight increase in the middle wing desired, I am not sure here, but it won't be big difference).

So the configuration is not better than evenly distributed lift in any flight regime.

Thanks for the answer: 1.FYI,  not talking about form drag.  That would be another question, reducing the span of a triplane, vs a monoplane.

So, if I need 1.5cL at takeoff, and 0.5cL during cruise, it would be better to have 3 wings at .5/3, or .17cL, than 1 wing at 0.5cl. Is that correct?

@Fred, it would be better to have 3 wings at .5/3, or .17cL, than 1 wing at 0.5cL given they are all the same span. Taking the three wings and putting them side-by-side to get a monoplane with 3 times the span is better still, because it removes the interference.


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Would wing fences or chord wise corrugated tape increase the efficiency of a short span wing?
This question is not a duplicate ...
Would wing fences or chord wise corrugated tape increase the efficiency of a short span wing?
I understand that larger span wings have less % span wise flow.
Which begs the question.....  at least conceptually, couldn't you just use end plates and multiple wing fences to reduce span wise flow?
or even better, use some sort of corrugated tape, with corrugations running chord wise, like the corrugated skin of a blue whale?

Induced drag cannot be reduced by forcing the boundary layer in a certain direction. Please do yourself a favor and use the search mechanism. This will lead you to a lot of answers which will help to clear the basics. With a better basic understanding you will see that this really is a dupe.

Sure, but we're not talking about induced drag here at all. Spanwise flow would reduce lift, so any reduction in span wise flow, would increase lift and increase the efficiency of the wing, no?

No. You cannot trick physics. The flow outside of your corrugation would still flow where it likes: away from pressure, towards suction. Always. The corrugation would increase drag. On the old Junkers designs it added 20% friction drag. But they saved induced drag by making the aircraft lighter. I still think we are talking about induced drag, by the way.

Well.... scrapping the corrugated wing idea, couldn't you use wing fences to reduce span wise flow?

Yes, you can. On swept wings that makes sense. Follow the link and learn how different that application is compared to what you have in mind.


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Why does Canada recommend a minimum rate of climb for ultralights of 300 ft/min?
What is a safe minimum rate of climb for ultralights? I understand most ultralights climb at least 300-800 ft/min and in fact the Canadian ultralight specs recommend a minimum rate of climb of 300 ft/min. Even the Lazair's specs note a rate of climb of 400 ft/min.
Why does Canada recommend 300 ft/min specifically? Why not 120 ft/min?  60 ft/min? Even at 60 ft/min, you could clear a 4' fence in 4 seconds. Of course you would want some clearance, so say 20 seconds. Seems pretty good in my book. Need to clear 50' trees? Just kite around in a circle.

IANAP, but circling at the end of the runway just to gain enough altitude to clear a tree sounds A) really dangerous, and B) really annoying for the pilot behind you waiting to take off.

That assumes you're taking off from an airport, and not your own "back 40". lol It also depends what speed you are flying at. If Lazair's stall speed is 18mph, and Vc is 1.3Vs, that makes it's minimum cruise speed about 24mph.

"Ultralights" is a very, very broad category when it comes to performance. What really bothers me is, why on earth would you want to climb as slow as possible? As quickly as possible might not be safe either, but there is a good reason why Vx and Vy are established and used!

@FreeMan Most airports wouldn't allow you to kite around to gain altitude as you would most likely be skimming the tops of the hangers....

Mentat answer : You decide how many fatalities you can happily live with and set regulations that are unlikely to make you too too unhappy in a bad year. | People are always going to die. If your assigned task includes limiting death rate to an acceptable level then limiting how deathly a death trap you are going to permit seems wise.

If you pointed me at an aircraft that couldn't climb faster than 200ft/min to save its life, I don't think I'd want to get in it.  You're basically in a glider at that point, but with the misleading existence of an engine making you think you're in a powered aircraft with capabilities it probably doesn't have.

Technically, Canada requires powered gliders to climb 360m in 4 minutes (Canadian Aviation Regulations section 522.65) and Very Light Aircraft to "have a steady climb gradient at sea level of at least 8.3 percent for landplanes or 6.7 percent for seaplanes and amphibians" (section 523-VLA.65)

Note for people confused like me by some of the discussions/answers, what the US calls an "ultralight" is a lot smaller than what Canadians tend to use the word for.

In a nutshell, in Canada, any plane can be flown with an ultralight license: both certified and non-certified ( amateur built) as long it meets the definition of an ultralight: max 1200 lbs gross, min stall speed of 45 mph.  True ultralights are divided into 2 categories, basic and advanced.  Basic is like the US version, except no speed or height limitations. You must wear a helmet and any passengers must be pilots. Basic ultralights must be registered but don't need final inspection. Advanced Ultralight is like the LSA category in the US, just no speed or height limitations.

When you fly gliders you discover it's quite common to run into air that's descending at 1-200 fpm, or "sink" in soaring-talk.  Descending air next to a thermal, or air descending due to downsloping terrain.  It's a lot more than that at times, but a couple hundred fpm is typical.
On a day where there's any convection (with rising air, there is always equivalent descending air adjacent to it) you are in sink quite a lot.  A 300 fpm ROC provides a reasonable assurance that you will be still able to climb while in most (but certainly not all) conditions of descending air, barely.
If you have an ultralight that can only climb 100 fpm, this might be fine on a smooth day with stable air, if you're patient.  If there is any vertical motion in the air however, you have a problem.  The minimum climb rate requirement is an attempt by the regulator to ensure that that average ultralight buyer will have a machine that won't kill him because it couldn't even out-climb a bit of subsiding air at the end of the runway.

Upvote but I think 100-200 fpm is an understatement. In my experience localized 500 fpm sink is extremely common - would expect to see that at some point on most flights - and I've run into 1500+ fpm several times.

Well yes but you generally don't fly ultralights on those sorts of days with the low mass and low wing loading.  It's quite unpleasant.  I'm talking about the mild thermic or subsidence conditions that someone flying an ultralight in the morning or evening, or a quiet day, might encounter.

@pericynthion : but that 500 fpm sink doesn't reach all the way to the ground, just as thermals don't reach all the way to the ground. From my gliding experience, it was extremely uncommon, in a flat area, to have any thermals whatsoever under 200 meters (strong enough to at least cancel the sinking rate of the glider). On hilly terrain it's different, but you should have ample height reserve when crossing over ridges in an ultralight anyway.


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Do ultralight engines need a thrust bearing?
Do ultralight engines need a thrust bearing?
Most gas engine applications only have a radial load ( perpendicular to the shaft).
Aircraft, not only have a radial load but also an axial prop thrust load pushing/pulling the shaft out/into the engine depending on whether the prop is a tractor or pusher prop, which usually requires a different kind of bearing able to take a thrust load.
So...do ultralights need a thrust bearing?
If so...
I assume the bearing of the output shaft of a reduction drive would have to be a thrust bearing.
However, we see 1/2 VW engines being used in direct drive applications, and I don't think they change the final bearing into a thrust bearing.
The reason I'm asking is, that I'm thinking of using a $400 15hp 4 stroke industrial engine for an ultralight application, just as we see in Rutan's quickie(?), ultralights using the vanguard brigs and stratton engines, and M Columbine's new design ( designer of the Cri Cri). Yes they are heavy, but they are cheap, 4 stroke, a TBO of 2,000 hrs, and are rated for continuous duty.
I plan to run at 75% power, which is about 2,500 rpm so I really don't need a reduction drive.
If I need a thrust bearing,  I could have a reduction drive with something with a 1.1 reduction. I could also have just a support thrust bearing bolted directly to the engine.
If I don't need a thrust bearing, then I would be off to the races.
I don't think the Lazair had a thrust bearing or a reduction drive when they used the non rotax engines.
So... do GA airplane engines need a thrust bearing, in the 15 to 30 hp range?

In automotive engines in general, the main bearings and front and rear end bearings are journal bearings, with near-zero end load tolerance (the end bearings have small flanges, but mainly for location tolerance).  I think if you look at the 1/2 VW conversions, you'll find they're building an entirely new crankcase (as well as cutting the crankshaft and camshaft, assuming they don't make those new), and they do in fact incorporate thrust-tolerant bearings in the ends of the case -- different bearings from the ones Volkswagen originally used in their four-cylinder air cooled engines (designed in the early 1940s and barely updated until the 1970s), which were specified for the intended use, with near-zero thrust loads.
If they didn't, even with only 30 kW in the minimal versions, the engine would have a very short running life.
If you're buying the engine already converted, you probably don't need to worry about it as this work will already be done for you, but if you plan to do the conversion yourself, be sure you look in detail at existing conversions, including their running life before overhaul in combination with the bearing type.

Any aircraft engine that spins a prop to generate thrust will necessarily contain a thrust bearing to prevent the propeller thrust force from pulling the crankshaft out of the block.  But since a bearing designed for mostly radial loads will also be capable of resisting a certain amount of thrust load, some automobile engines converted for aircraft use can get by without having a thrust bearing added to their crankshaft supports.

Most automobile conversions (and some ultralights) use a speed reducer, either gears or belts -- and the thrust bearing is on the reducer's output shaft; hence protecting the engine bearings from thrust loads.

yes yes. without it, the engine would quickly fail in an ugly way.

Actually, the main bearings in most engines, anything from a vertical shaft lawnmower type up to a racing V8, are journals, not ball bearings -- and they don't take end loads at all well.  In typical automotive engines, only the end bearings have any thrust load tolerance at all; the mains will have the crank rubbing on the block and cap if there's any thrust load.


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Why does the eurofighter have a variable engine inlet?
Does anyone know,  why the eurofighter does have a variable engine inlet while similar aircraft such as for example the rafale or the f-16, which also has an underslung engine inlet, do not employ any kind of variable geometry in their engine inlet?
My two own theories are, that it is either to primarily help with inlet performance at high angles of attack or to optimize the capture area of the intake for the wide range of possible airspeeds of the aircraft. Sadly I haven't been able to find any concret evidence to underpin either theory.
Given that the variable inlets bring with them quite a bit of added technical complexity, I would expect strong arguments to justify their incorporation in the design.

It is a design choice as the intended missions of both aircrafts differed slightly.
The inlet of the Eurofighter Typhoon features variable geometry, to improve the performance at high angle-of-attack and performance at transsonic and supersonic speeds. This design choice was made because the Eurofighter Typhoon is intended to be used as a top of the line, hyperagile aircraft which outstanding dogfight and interceptor performance. The twin-egine design, canards, relaxed stability/unstable design and the originally envisioned thrust vectoring engines were to help in this mission profile. The expensive and complex variable geometry inlet is therefore only a logical choice when you put in other equally complex systems to enable this use profile.
The F-16 on the other hand was designed to be relatively cheap, easy to maintain and to have a low operating cost. This was partially achieved by installing only a single engine with a straight inlet and no complex variable inlet duct. The disadvantage in the transsonic and supersonic regime as well as drawbacks at high angle-of-attack were accepted in order to keep the aircrafts cost low. Other fighters such as the F-15 or F-22 should fill the role of higher-cost aircraft.
See also this post about the inlet of the F-16.
In hindsight, the F-16 has obviously aged better. The Eurofigher Typhoon while being remarkably agile, probably the most capable Dogfighter ever, has never been in a real dogfight (at least to my knowledge). Instead, missiles have taken over this aspect of air-to-air combat. The F-16 on the other hand is a very cheap still very capable aircraft which is cheap to operate while it still can execute todays mission reasonably well. The Eurofighter can fullfill the same mission, however operating costs are significantly higher.
P.S. Do not get me wrong, the Eurofighter is a remarkable piece of engineering. Especially the flight control system is an absolute beast!

Additionally, the  "state of the art" in aerodynamics and engineering has improved significantly. When both of these aircraft were designed, it was thought that the only way to prevent supersonic shock waves from disturbing the flow at the compressor was with a variable geometry inlet. Since then, we have learned how to use fixed geometric shapes on the inside of the inlet that have the same effect. See this: https://aviation.stackexchange.com/questions/68320/how-can-an-f-22-raptor-reach-supersonic-speeds-without-having-supersonic-inlets

In hindsight, the F-16 has obviously aged better. The Eurofigher Typhoon while being remarkably agile, probably the most capable Dogfighter ever, has never been in a real dogfight (at least to my knowledge).

Can you elaborate on that more? As far as I know, the countries operating Eurofighter haven't really been in an air superiority war since it exists, what do missiles have to do with it?
It's not like other modern air superiority fighers like F-22s were ever in a dogfight - it's a function of history, not design or "aging".

@Mavrik I think we both mean the same thing. All I want to say is that the specific design advantage of the Eurofighter (its high agility) is not needed anymore on todays battlefield because of modern long-range and/or highly agile missiles. The F-16 on the other hand is still cheap. Therefore, in the end, the Eurofighters agility was never really needed but the F-16 "cheapness" was always a important factor. This is why I say, perhaps the F-16 proofed to be a more "successful" design. Of course, if history were different, this conclussion would also be different.

Also, for the downvoters: Please indicate why you are unhappy with this answer so that I can improve it. Thank you!


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What is the difference between EPE and HUL on the Satellite Status Page of GNS systems?
For exemple on the GNS430 satellite status page you can read, in addition to the GPS receiver status :

EPE (Estimated Position Error)
DOP (Dilution of Precision)
HUL (Horizontal Uncertainty level)

I found here explanations about DOP computation, and GNS430 manual states that:

"EPE uses DOP and other factors to calculate a horizontal position error, in feet or meters".

About HUL, the same manual explains that

"the Horizontal Uncertainty Level displays a 99% confidence level that the aircraft position is within a circle with a radius of the value displayed in the HUL field"

It seems to me that EPE and HUL are exactly the same thing and differ only by the confidence level. If it's the case, what is the confidence level of the EPE ? If not, what is the difference between EPE and HUL ?

EPE is the 1-sigma error level and thus represents the position error circle with a 68% confidence level. Reference: NPS


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Can you identify this unusual plane by its description?
I saw this plane many years ago:

Same shape and size as an commercial airliner.
No windows along the fuselage
Painted flat gray (like a naval ship)
Had a blue circle on its "nose" (Blue painted radome)
Rectangular "hump" with rounded corners on the bottom of the fuselage.

If you can identify this plane, could you please tell me the make and model and what it's used for?
UPDATE: the question has been put on hold so I will try to be more specific. I saw a plane fitting this description flying unusually low over the Chelsea neighborhood of New York City on the morning of September 11, 2001 shortly before 9:00 AM. It was flying south over 8th Avenue. I had just exited the subway station at the 23rd Street and 8th Avenue. I was walking west towards my office on 11th Avenue when I was startled by a very very loud sound directly overhead. I turned and looked up and saw this low flying plane. I have never ever seen an airplane fly that low except near an airport terminal. It was shocking and frightening. I literally thought the plane was going to land in the middle of 8th Avenue. I remember looking for smoke or visible signs of damage but it looked like it was okay. It was actually slowly and gradually descending, but then it climbed back up. There were no markings on the plane apart from the blue "bulls eye" on the "nose" (pardon my lack of knowledge about aviation. I don't know the proper terminology).
Several minutes later I was on the M23 bus when I heard an extremely loud bang, which I would assume was the first plane hitting the WTC. It was so loud that my ears were ringing for a while. I remember trying to convince myself that the sound had nothing to do with the plane because the thought was too awful. At the time, there was a lot of construction going up on west Chelsea (gentrification. high rise luxury condos being erected where transsexual prostitutes used to roam the streets). I tried to convince myself that it was from the construction. The image that came to mind was taking a steel garbage dumpster and throwing it off the top of a skyscraper.
I don't know what this plane was but it had something to do with what happened on 9/11. If only I had had a camera at the time and could have taken a picture. I need to know what this plane was and how it fits into what happened that day. This is not a joke. I realize that this is going to be controversial and some people may be upset. I want to reiterate that this is not a joke of any kind. Please do not flag this post. I need an answer to what has haunted me for 17 years. Thank you.

Several military aircrafts are designed based on commercial aircrafts. You may refine your description as it fits at least the A330MRTT, all C135 versions, KC767, KC46 and many other listed here. Do you have an idea of which air force was operating it? what other characteristic can you provide?

A photograph of the aircraft would be useful for identification as this description can match multiple types.

2 or 4 engines? Prop or Jet? T-tail or conventional? Conventional gear or tricycle? Was it headed north or south at the time you saw it?

I think 2 engines. It was a jet. It was headed south.

@ManuH: From the date we can rule out the Airbus and the KC-46. With two engines neither the C-135 fits the description. The rectangular hump was a phased array antenna. There are many SIGINT/ELINT versions of jets around, some of them part of black programs. This was one of them.

Sounds like it may have been a Boeing RC-135

(source)
No windows and a flat grey color would lend to either a cargo aircraft or a military aircraft with military being likely considering the color choice.


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What is the "compression ratio" of air hitting the nose?
Sorry but I don't know how to accurately phrase this question. I want to know how much compression results from air hitting the nose of an aircraft.
I'm interested in the traditional airliners (Mach 0.85, blunt nose, like B747 or A320), as well as the supersonic stuff like Concorde and even Mig-31.
Is there an equation for this? Presumably with cross section area and some kind of coefficient of form drag. I'm looking for a density and/or pressure change.
I want to know if this is significant, like if the aircraft actually has to do a lot of extra work to overcome the greater density in the nose air. But there are actually several reasons. I want to calculate how much heat arises from the adiabatic compression, which will let me know if some kind of active cooling is required for a given material. I want to know if the increased density of the nose air actually changes the general drag equation, like if you have to put in a higher density value when just calculating the fuselage drag. I want to know what possible boost you can get from the "ram effect" such as from a ramjet.
In this question, all I'm really asking for is the compression ratio...or change in density, or change in pressure...sorry I don't know which one is more accurate. If I just have that, then I think I can answer the rest using other equations I know of.
I wanted to explain myself in case this appears too broad or too xyz thing, sorry can't remember that term either. Hopefully someone will know an equation that can answer this. Equations give hard numbers, so they're always better than relative adjectives like "larger", "negligable", etc.
Also, if someone can suggest tags, please do so. Couldn't think of anything better than "drag".

The pressure on the nose (or in the pitot) is called total pressure, and includes pressure due to velocity and static pressure. The compression ratio is the ratio between this total pressure and the static pressure. Subsonic and supersonic velocities have different effects. See this document which is quite complete.

I think what you seek is called the stagnation pressure, which is the pressure rise experienced in a given parcel of air if all the kinetic energy associated with that parcel's velocity were to be converted without losses into potential energy (stored work) of compression according to the ideal gas law. The equation for stagnation pressure is
$$ \frac{P_\text{stagnation}}{P_\text{static}} = \left( 1 + \frac{\gamma - 1}{2} \times M^2 \right) ^ \left( \frac{\gamma}{\gamma - 1} \right) $$
where $\gamma$ is the ratio of specific heats for air and $M$ is the Mach number.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

edited to include equation. -NN

OP did not specify stagnation pressure in an engine.

I'm interested in air hitting the leading edge of an object, like the nose or wing edge. I don't see how to use the equation, tho, because it doesnt have any cross section area or coefficient of drag. Surely the nose of a B747 has more stagnation pressure than the nose of a tiny business jet at the same speed? Likewise for the leading edge of a wing versus the big nose. Without cross section area I don't see how it can be trustworthy. Maybe stagnation pressure is not the right thing after all?

Also, the edited equation was wrong. Exp means 10 to some power, but what wikipedia says on its page for stagnation pressure, just shows a plain exponent. Edited it.

@DrZ214 Looks like you’re looking for a drag force then.

@Koyovis No, that's a force. What i want is a pressure change or density change, but it's obviously highly related to drag. Maybe what I'm thinking of is some kinda cumulative or average pressure change for all the leading air, because it seems stagnation pressure is just measuring pressure at a single point.

@DrZ214: Stagnation pressure can be found at only one point (the stagnation point, obviously) so there is no area on which it acts. Pressure drops when you move away from the stagnation point, so in reality you will have to integrate with a pressure gradient when calculating drag on the aircraft nose. But stagnation pressure is real: It limits the top speed of blimps to the point where internal pressure equals stagnation pressure, or the blimp's nose will deform.

@DrZ214 The stagnation pressure is the maximum pressure on a moving blunt object. It's basically the (infinitesimally small) point where air doesn't "know" whether to go left or right around the blunt object, so instead it just compresses. The rest of the air will have a lower pressure (intuitively, because it can convert some of its kinetic energy to sideways kinetic energy instead of potential pressure energy). How quickly the pressure drops away from that point is of course a function of all the factors you mention.

@DrZ214 So, you’re looking to quantify what happens to the air in front of a body subjected to airflow, correct? How much energy it costs to get out of the way?

@Koyovis No, I'm looking for a pressure change or density change. Maybe energy can be calculated after that? Or from that? I don't know. If you know some equation that involves energy along with one of those things, go ahead and post it and it should be solvable for the variable I'm interested in. The problem with stagnation pressure equation is it's just at one point. Like Peter Kampf said, you can do it with a pressure gradient and then integrating it. Maybe there's an equation for a pressure gradient, but I don't know it is.

@PeterKämpf Is there an equation for the pressure gradient across some surface at some airspeed v, and ofc cross section area A? Ideally a body of revolution like a sphere, sears-haack, ellipsoid, etc. I hoped that just the Cd number would be enough, but if it's not that simple, oh well. But if an equation exists for the gradient, then I can definitely integrate it one way or the other. It's okay if there are different equations for different shapes.

@DrZ214 what you are asking is essentially a solution for the pressure field, which exists in analytical form only for particular cases like potential flow.

The simple answer for sub-sonic aircraft is zero. Air does not compress significantly until shock waves start to form.
Pressure changes with speed (as per Bernoulli's principle) but density can be treated as constant.


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Why doesn't condensation form on the inside of airliner windows?
If outside the aircraft is pretty cold and inside the cabin the air temperature is different, why don't the windows become fogged up on the inside?

Very low humidity really helps.

You didn't explicitly say it, but I assume you're asking about condensation on the inside? Condensation forms very commonly on the outside, when airliners descend from cold air into warm, moist air close to the ground.

@Pondlife yes, condensation inside the cabin.

Who says it doesn't?  I've seen it, though rarely.  I suspect, though, that the windows are double paned, for insulation, and condensation only happens if moisture somehow gets between the panes.

The humidity at the altitude airliners fly at is VERY low. However, a tiny bit of moisture does build up - if you look closely you'll see little ice crystals on the window (it's also very cold up there - in the negative degrees F usually). On top of that - airliners have multiple air conditioning "packs" that dehumidify the cabin air so there would be indeed by very little moisture to condense on the inside of the windows. Also, if you look at the windows - they are often two or three layers with quite a bit of space separating the outside of the window from the inside (often multiple inches). Some of this is for pressurization and some of this is for thermal insulation. The VERY cold outdoor air is not in direct contact with the inside cabin pane.

The OP is asking about condensation on the inside of the windows

I have no answer other than that there are air conditioning packs in the plane that run constantly and that dehumidifies the air, thus internal condensation would be limited in the same way that it is on your house windows.

"Condensation would be limited in the same way that it is on your house windows": The conditions are not the same, a small cabin volume for several hundredths of breathing people can't be compared with a house volume for, say, four persons. When the aircraft is in clouds, air should be very damp. Packs seem to extract water from air (4. Pack operation and 5. Water extractor).

@mins: The packs handle bleed air only. The very first picture of the linked answer shows that at some point, air from packs and from "recirculation fans" is mixed.

@sweber: You are right, only bleed air, which could be damp air from a cloud unless bleed air from the compressor section is dehydrated before reaching the pack. In addition water from breathing is collected in the cabin and recirculated. That's my understanding.

@mins: Consider the difference in condensation between a antique single-paned house window, and modern  double/triple paned windows.

@jamesqf pretty much what I was saying

@jamesqf: I'm ok with that. It seems more accurate to say that there is no condensed water because the windows are well insulated, than because air is dry.

Double glazing.
The gap in between the panes has very little moisture in it, so there is nothing to condense on the outside pane.
There is no condensation on the inside pane because of the temperature gradient between the panes.

Outer window panel is pneumatically isolated from the  outsife, it is a pressure window , the second and third inboard windows are primarily there for thermal and acoustic insulation. Condensation (frost) forms on the inside of the outer pressure window because there is a pneumatic pathway to the cabin and passenger breath contains water vapour which becomes attached to this surface during the flight.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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Can passengers tell (without GPS etc.) where the aircraft is heading to?
I was watching Mayday Varig Flight 254 and at minute 38 it said that some passengers noticed that the plane was flying in the wrong direction. This accident took place in 1989, so my question is: without 'modern' aids like GPS, smartphones, in-flight entertainment system maps etc., can a passenger really notice where the aircraft is heading to?
(I suggest you watch that minute of the video if you have the time. It will help to understand the question.)

Some aircraft have moving map displays on the in-flight entertainment systems. Others have ATC audio that you can listen to, and then there are windows and prominent landmarks they can see, so I would believe that yes, its possible for a passenger to figure out which direction they are flying.

Yeah that's right but now. In 1989 there weren't those screens with maps

There were still windows and land features...

Yes, but in the amazon rainforest? Almost at night? At a lot of feet AGL? I don't know

I myself carry a compass when I fly overseas.

@kepler22b - According to Wikipedia, the flight departed at 17:45 from Marabá. During that time of year, sunset is roughly around 18:15. So they still had 30 minutes prior to sunset. I can say anecdotally, that if I had daylight and departed from my home airport on a familiar route, I believe I would notice that we were heading the wrong direction. Interesting question!

if the passenger knows the terrain well enough he can.

I've been on a night airline flight where I could see stars out the window. If someone had some knowledge of the stars and could see them, they could figure out the direction the plane was headed.

in the video they mention the position of the sun, some passengers are attentive to this kind of "details"

For an aircraft taking off half an hour before sunset, that sunset tends to be fairly conspicuous.

@FredLarson Can they see where they are relative to the stars? How can they do that? The stars are "moving"

@kepler22b: A plane window gives a very small view of the sky. If I'm sitting on the right side of the plane, and out my window I can see the Big Dipper and Polaris, clearly I'm looking north and the plane is headed west. This is not terribly difficult.

@FredLarson I'm one of those attentive types, not because I'm worried, but because I'm interested and also a geek, always challenging myself to work out where I am from the sun, stars, big cities (even at night, many are very distinctive), bodies of water etc.  I agree, it's not difficult.

@Simon: Yeah, it's challenging for me being a passenger in a plane. I always want to know our airspeed, altitude, and heading. At least I can get an idea of the heading by looking out the window.

@FredLarson Oh. Now I see. Anyway, it was before sunset, so perhaps they couldn't really see the stars well

@kepler22b: They might have been able to see that one really big, bright star near the western horizon! 8v)

Well, on one flight I was on, I could tell the direction in the landing pattern because I had studied the destination airport diagram and I knew the basic flight heading of the flight.

can a passenger really notice where the aircraft is heading to?

A wide variety of observable phenomena and general attentiveness can lead to an approximate understanding of which direction the aircraft is heading in.
Passengers can rely on visibly observed phenomena such as stars, moon, sun, ground topology, or major landmarks (interstates, cities, facilities etc.). General familiarity with flight routes and runways, coupled with attentiveness to significant changes in roll, pitch, and yaw can bolster an understanding of the flight's direction.
Items such as a compass (built into Casio calculator watches?) could also be used to get an approximation of direction. They may be affected by other instrumentation and mechanical features along with being really cheap.
Some passengers will be able to note which runway they took off from and a divergence from a typical path. While still ascending they can more easily notice landmarks that are likely familiar, and recognize that no significant change in direction has happened since orienting themselves.
It's possible too that cognitive bias may be another explanation. The media, and documentaries like this, seem to embrace an event as predictive or accurate despite a long history of inaccuracy or patterned behavior. That passenger who makes 98% incorrect assertions of plane direction is only recorded or listened to after the fact and appears to be 100% accurate in their vastly under sampled assertion set. How does it go? A broken clock is right twice a day?
Also, thanks to all the comments, tried to cover most of those notes/ideas here.

Absolutely you can. I have been doing this since the 1980s when I started flying. I love maps and love looking at the earth from 30000 feet up. When I see interesting features I want to know what they are and where they are. Nowadays with google maps it's very easy to do this without getting into a plane but back in 1989 your best opportunity to see the earth from up high was a plane flight. Every trip for me was an opportunity to explore and I'd prepare for the flight by having a look at a map and gaining an understanding of the significant landmarks we'd be flying over and the potential routes.
When you know what to expect and what things generally look like from altitude it's easy to keep track when over land. Working out the exact heading is trivial if you can see the sun or a few key stars. Most plane flights are above any clouds giving you great astro-navigation cues (learning how to determine compass directions from the stars is easy in both hemispheres).
Once you progress to being a frequent flyer you tend to know exactly where you are. I've flown Sydney to Cairns, Auckland and Melbourne enough times now that I can pick out all the cities, towns, rivers, lakes, damns, harbours, bays, open cast mines, wind farms, power stations etc. Over the ocean it's just dead reckoning (ssw for 2.5 hours from Sydney... should see the NZ coast soon. oh yeah there's the entrance to Kaipara harbour).
I would suggest that there is a subset of the passengers on board who know exactly where the plane is currently and where it is heading. They're the ones in the window seat spending the flight staring out the window like they think the view outside is more interesting than the movie....

There is a way to determine North with your wristwatch, if the sun is visible through the windows (see here). This would help to notice if the plane is heading in a completely wrong direction. Many hikers know this trick.
Of course, it is even more obvious if the sun is ascending in the morning on the  completely wrong side.
If it is in the night, many people know where the North star is, and some more skilled in astronomy may guess its location even if they see some other piece of the stellar sky.
All this is not likely for a single random chosen individual, but if there are say 500 passengers on a plane, it may a hiker between them, and it may be an astronomer between them because even finding a certified pilot between passengers is not completely unheard of.

authors: Вячеслав Передерей


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What is the purpose of a Traffic Pattern?
I always see images with a runway and a rectangular flight path over it and ask myself why do the pilots take off, fly around the runway and get back and land, what is the purpose of doing that: take off, fly parallel to the runway and land? Am I missing something?

Source

It shows the different things that pilots can do in the pattern. Notwithstanding a student pilot in some part of his or her training will go around the pattern doing "touch and go"s.

You might think of the traffic pattern as similar to an automobile round-about or traffic circle.

I think there are two parts to your question: why do traffic patterns exist, and why do people fly around them in circles?
First, a huge amount of aviation effort goes into avoiding collisions, for obvious reasons. One part of that is making everyone behave as predictably as possible, so that you know what to expect from other pilots and where to look for aircraft near you. That's why there are regulations on airspace, altitudes, speeds and all sorts of other things that affect how and where aircraft fly: if everyone is following the same rules then life is much easier. For example, if you're driving down a one-way street in your car, you can be (reasonably) confident that you won't meet someone coming the other way, and drive accordingly.
Because aircraft operate very closely together around airports, there's a higher risk of collisions and it's especially important to have very clear rules for how and where pilots take off, approach the runway and land. The traffic pattern is the aviation equivalent of road markings: it 'shows' where you should fly in order to fit in with everyone else, fly in the same direction, be visible to others, and generally be as obvious and predictable as possible.
Second, why do pilots take off, fly around the pattern and land again? Usually, because it's a great training exercise. In the process of going around the pattern you have to perform multiple important maneuvers and tasks, including:

Taking off
Climbing out in a controlled, precise way
Making coordinated turns
Managing airspeed
Operating key aircraft systems like the landing gear, flaps, lights etc.
Using checklists
Working with ATC (if it's a controlled airport)
Descending in a controlled, stable approach to land
Landing

Since taking off and landing are often considered the riskiest parts of a flight, doing pattern work is a great way to get as much practice as quickly and easily as possible. For example, I just got back from doing some night pattern work: I did 10 takeoffs and landings in about 35 minutes. That packs a lot of practice into a short amount of time, which is useful for everyone but especially for student pilots.

To improve your answer, mention that traffic circuit to a certain degree also exist for noise abatement reasons, at least where they are depicted on the Visual Approach Chart (here in Europe). Traffic circuits usually are away from housing areas and also provide obstacle clearance to a certain degree.

@SentryRaven From my perspective, I would not say that any standard rectangular traffic pattern—such as the one in question—exists for the purpose of noise abatement, though noise abatement is a consideration in determining whether the pattern is a right or left pattern; mine may be an FAA dependent perspective. However, I would also question whether charted visual approach terminal procedures fall within the scope of this question.

@SentryRaven That's a good point, although I tend to agree with JonathanWalters: noise abatement isn't really a consideration in a standard pattern layout, apart from sometimes using a right-hand pattern instead of a left-hand one. But it is certainly a reason for some non-standard patterns to be laid out the way they are.

Then this must be really an ICAO thing. Traffic circuits in Germany are influenced a lot by noise abatement.

@SentryRaven Well, they don't sound "standard" then. :)

@Lnafziger "Standard" patterns are only used at controlled airfields, i.e. they are not depicted on the VAC. Uncontrolled fields in Germany always have the pattern depicted for guidance on the VAC.

@SentryRaven That's my point.  The picture that he shows (and the question that he asks about) depicts a standard traffic pattern.  You have a valid point that traffic patterns are sometimes modified for noise reasons, but it really has nothing to do with why we fly have a standard pattern in the first place.


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What are the ways to keep the birds away from airfields?
I know bird strikes are a major issue for many airports, and that measures are taken to keep birds away.
There are some obvious things airports can do like not to have any stray garbage, food, etc. in the open, but I'm sure other less obvious methods are also used.
What exactly are some of the widely used techniques to keep birds away from airports?

Somebody gets to fly falcons, hawks, and an eagle for a living: Feathered fighters defend Pearson Airport’s skies (2011)

Near PDX there are fields that Geese like to hang out in. They erected short "fences" of black plastic sheeting, only about 2 feet tall, that run east-west through the fields. I'm told the Geese won't land there because it hinders their vision. Here's a Google maps link that shows them. However, now that I look at the map, I wonder it's this is real - Why doesn't the airport itself have them?

The FAA has an FAQ on bird strikes and other wildlife hazards, which includes a (broken) link to their Wildlife Hazard Management at Airports - A Manual for Airport Personnel document.
Chapter 9 is called "WILDLIFE CONTROL STRATEGIES AND TECHNIQUES AT AIRPORTS" and includes the following techniques:

Change flight schedules
Modify the birds' habitat (food, cover, water)
Physically exclude the animals (not so easy with birds)
Repel the birds with vehicles, chemicals, sound, visual objects (flags, scarecrows, fake dead animals, lasers)
Repel the birds with trained falcons or dogs
Shoot nonlethal projectiles at the birds
Trap, remove and release the birds at a different location
Kill the birds with poison or other means

The document summarizes the various techniques like this:

Habitat modifications to minimize food, cover, and water and physical
  barriers to exclude wildlife are the foundations of wildlife hazard
  management programs for airports. In addition, an integrated array of
  repellent techniques is necessary to disrupt normal behavior and to
  stress hazardous wildlife that attempt to use the airport. These
  repellent techniques must be used judiciously and backed by real
  threats to minimize habituation. To this end, lethal control of
  selected individuals of common species is sometimes necessary to
  reinforce repellent actions.

about.com has a good article which lists the many methods that are used to scare birds away from airports. It includes:

Modify the habitat: Remove sources of food, such as seed-bearing plants, use insecticides to eliminate insects as a food source, remove bushes and plants that can be used for nesting
The use of visual devices (lasers) or audio (sonic cannons, recordings) to scare away birds
Trained falcons, dogs, or other predators to make the airport inhospitable to birds.

Another source: Birds at Airports - Internet Center for Wildlife Damage Management

(Image Source: WikiMedia Commons - Author: Bluescan sv.wiki)

Lasers at an airport? That sounds like a recipe for disaster

@raptortech97 I was skeptical at first as well, but they do exist: http://birdcontrolgroup.com/aerolaser-groundflex/

They don't make the birds believe the area is inhabited by predators, they simply make the area inhabited by predators. I remember I read interview with local bird control worker where he said he always lets the hawk kill the prey, otherwise the birds would not fear it.

@JanHudec That's what I wanted to say. How would you rephrase it in my answer?

Not sure; I think the part of the sentence is not needed.

@JanHudec So, how do they keep the falcons out of the flight paths? It seems like a falcon or a hawk would be much more of a problem for a jet engine (or any other part of an aircraft) than your average song bird.

@reirab These are trained animals that return when called. "DLH123, caution wake turbulence, landing falcon" ;)

@SentryRaven Do they call them back on the tower frequency? "Falcon 45H, cleared to land 27L."

On a visit to Blackpool Airport control tower a few years ago, they mentioned that they flew a hawk/falcon (not sure which) every day to deter birds

Peregrine falcon would probably always be included as it is best at hunting flying prey, but most likely they have several birds of different species and send the one appropriate to situation.

The airport authorities use a number of techniques to manage the hazard of bird strike in and near airports. These include,

Elimination of bird habitats- Birds usually come to the airport area due to the availability of food sources. Denial of food to birds by eliminating the sources (like landfills, garbage, crops, dumps etc) will reduce the chance of birdstirke by eliminating the source.
It also helps to keep the airport and surrounding areas unattractive to birds (for example, by eliminating water stagnation, vegetation cover etc) as it prevents the birds from nesting there.

Scaring them away- A number of airports use a number of bird repellents like pyrotechnics, cannons (mostly just prior to aircraft approach to prevent bird habituation), chemical repellents etc.
In some cases, specially trained animals (like falcons, dogs etc) have been used to scare away the birds or they are trapped.

In some cases, the flight schedule can be altered if bird movement is expected in the area (for example, if the airport is on a bird migration route).

The problem can be solved by simply killing the birds in the area, though this drastic step is usually not implemented except in the most extreme cases. In general, the nesting sites of birds are destroyed to prevent them from staying permanently near the airport.

Usually, a combination of above methods is used to manage the birds.
The only procedure available for flight crews is to either delay the flight (or land in alternate runway if its available) in case birds are detected over an area. This however, is dependent on a number of variables like fuel availability, flight schedule etc. There are some radar systems under development that could detect birds; however, these have not reached operational stage yet.

Trap, kill, harass, or deprive them of food and shelter. The number of bird strikes at my airport has been reduced dramatically by consistently harassing the birds to the point where the birds eventually adapt and learn.

All the correct information has been added except one point. The engines nose spinners have a design painted on them. Once the engines are running the design is suppose to look like the eye of a bird of pray to scare off birds in flight.
  Does it work I can not say but they claim it does.

"they claim" who?

Related question... Wikipedia, and apparently Rolls Royce, say so.

So that's what those are for.  I've always wondered...


text:
Is there an issue with an acceleration followed by a deceleration when flying a SID?
Sometimes I see SIDs with a long straight climb segment (say 10 nm), followed by a wide turn with an IAS constraint (e.g. MAX 210 KIAS).
Is there any issue in accelerating to 250 KIAS and then decelerating shortly before the turn? Is that ever done?
EDIT
Fictitious example:

EDIT #2
Here's the actual chart (I hope no copyright infringement):

I'm referring to LIPY, but I'm not able to find a link w/o a registration (I fear that I cannot post the chart). I add another chart in my post with an example.

In theory it is allowed (see nulla poena sine lege), but no reasonable pilot would do so.
The purpose of the IAS limitation on the turn is to limit the radius to keep aircraft within protected airspace. There is no such issue on a tangent segment.
However, it is a waste of fuel to speed up and then slow down within a few miles, and at best that will save you a few seconds of flight time. At worst, it creates a risk that you won’t slow back down in time for the curve, which could get you killed.
So, pilots will fly the first tangent segment at or below 210kt, and then accelerate once established on the second tangent segment.

The usual chart verbiage is, “do not exceed 210 KIAS until passing FIXXX”. That precludes speeding up and then slowing down.
While doing that & then slowing for the turn would probably maintain obstacle clearance (by keeping the turn radius to an expected value), you might compress with traffic ahead that is flying the expected profile. Also, some departures direct the slower speed to keep you climbing and minimize the noise impact to houses along the departure route.

Please, take a look at the EDIT #2 for the actual speed restriction.

@Cristiano The distance shown in your example isn't enough for much of anything without an afterburner to climb to about 3,000' AND accelerate to 250 knots and then to slow back to 210. An F-15 probably could... but, why?  In normal operations, what you suggest isn't done, at least in any departure I've ever seen.

If there is a problem with compression with a previous departure, the controller should issue a control action to keep the required longitudinal separation.